The present invention relates to synthetic rigidin analogues as anticancer agents, to salts, solvates and prodrugs thereof, and to a method of producing the same.
Cancer is the second leading cause of death in the United States. Cancer is a class of over 100 diseases that is characterized by unregulated, abnormal cell growth and proliferation. Anticancer agents are among the many different tools used to battle cancer. They can be used solely or in combination with other treatment regimens to fight the disease. Cancer rates are only expected to rise in the future as the average life-span continues to increase and thus the development of new anticancer agents is a top priority in the drug discovery community. Of particular importance is the vast number of new anticancer agents that have been derived from natural sources. Out of all new anticancer agents developed between 1981 and 2006, only about 20% have been totally synthetic and thus new natural compounds have served as excellent sources to create more potent, more selective agents than those that are currently used in the clinic.
Plant-Derived Natural Products as a Source of Anticancer Agents
Plants have a long history of use in the treatment of cancer, dating back as far as the 4th century BC when Hippocrates of Cos used oil obtained from the daffodil Narcissus poeticus L. to treat uterine tumors. Several plant-derived compounds are among the top anticancer agents used clinically; these include the vinca alkaloids, vinblastine (FIG. 1) and vincristine, and the taxanes taxol (FIG. 1) and its semi-synthetic analogue docetaxel. The vinca alkaloids are among the first plant-derived anticancer agents to advance to clinical usage and were first isolated in the 1950s from the Madagascar periwinkle plant.
Many other plant-derived compounds have been used as leads in the discovery of more potent and less toxic anticancer agents. Included among these are podophyllotoxin (FIG. 2), an antimitotic isolated from plants of the genus Podophyllum, and camptothecin (FIG. 2), a topoisomerase inhibitor isolated from Camptotheca acuminate, a tree extensively used in traditional Chinese medicine. Podophyllotoxin and camptothecin failed clinical trials themselves due to severe toxicity and poor water solubility but they both served as lead agents for the development of several clinically used anticancer drugs. These include three semisynthetic derivatives of podophyllotoxin teniposide, etoposide (FIG. 2), and etoposide phosphate, as well camptothecin analogues topotecan (FIG. 2) and irinotecan, all of which are now used clinically for the treatment of a variety of cancers.
Marine-Derived Natural Products as a Source for Anticancer Agents
In addition to plant-derived anticancer agents, a relatively new resource for the discovery of new anticancer drugs is marine natural compounds. These include natural products isolated from aquatic fungi, cyanobacteria, sponges, and tunicates. Cyanobacteria has been a rich source for new bioactive agents yielding approximately 300 new alkaloids, which are likely produced as toxins to ward off predators. Marine alkaloids derived from cyanobacteria have been well characterized and have shown various mechanisms of action against a variety of cancer cell lines, which include cell cycle arrest in G1, targeting of actin or tubulin as well as histone deacetylase (HDAC) inhibition to list a few. Fungi generally produce a wide array of secondary metabolites that have shown promising anticancer potential, however the mode of action (MOA) of many of these alkaloids is not fully understood.
Marine sponges and tunicates have proven to produce some of the most interesting and useful marine-derived alkaloids to date. Many of the identified compounds have reached clinical trials and have very well defined MOAs, many of which are similar to those of the plant-derived natural products taxol and the camptothecins. These include hemiasterlin and its synthetic analogue E7974, which display a potent anti-tubulin activity, and the makaluvamines, which inhibit topoisomerase II (TOP II). Tunicates have long been known to be a major source of compounds for the treatment of cancer. Among these, the lamellarins and related pyrrole-derived alkaloids stand out as a group of marine alkaloids with promising biological activities, such as cytotoxicity, reversal of multidrug resistance (MDR), HIV-1 integrase inhibition, antibiotic activity, human aldose reductase inhibition, immunomodulation, antioxidant activity and others. In light of their fascinating novel structures, intriguing biological properties, and the difficulty in obtaining large quantities from natural sources, marine pyrrole-derived alkaloids have attracted considerable attention from organic and medicinal chemists. At present, several marine natural products are in clinical trials and one drug, trabectedin, was recently approved in Europe as the first ever marine-derived anticancer agent.
Rigidins as Potential New Anticancer Agents
Although there has been intense investigation into marine natural products, there are still many very potentially useful medicinal agents that have not been researched fully. Among these are the marine alkaloids rigidins A, B, C, D (FIG. 3) and E isolated from the tunicate Eudistoma cf. rigida found near Okinawa and New Guinea. The rigidins, due to their low availability from natural sources, are poorly investigated natural products even though they have several interesting biological properties. They are reported to exhibit cytotoxicity against cancer cell lines, while rigidin A was also shown to possess calmodulin antagonistic activity.
The rigidins incorporate the pyrrolo[2,3-d]pyrimidine ring system, which is considered a “privileged medicinal scaffold” (PMS). PMSs are molecular frameworks that are seemingly capable of serving as ligands for a diverse array of targets. The pyrrolo[2,3-d]pyrimidine ring system is analogous to the purine framework and, in addition to the rigidins, it is also a common motif in several other natural products, such as the nucleoside anticancer antibiotics tubercidin, toyocamycin, sangivamycin. Thus, the rigidins are an unexplored class of marine alkaloids that have a high potential for multiple biological activities.
Previously, the inventors discovered a three-component chemical reaction leading to an efficient synthesis of marine alkaloids rigidins (FIG. 4). Although these natural products had been reported to possess weak antiproliferative and calmodulin-antagonistic activities, their biological properties had been scarcely studied due to the insufficient quantity of material available from isolation and a lack of an efficient chemical synthesis. The inventors developed an efficient synthetic approach, which allowed them to prepare sufficient amounts of material for biological studies.
Evaluation of these synthetically prepared alkaloids against several cancer cell lines revealed very weak antiproliferative activities (>100 micromolar IC50 values). The current discovery deals with the finding that an alteration of this reported synthetic methodology to a novel four-component reaction leads to the preparation of rigidin analogues possessing a previously unknown hypoxanthine-like skeleton (see the three representative, possible compounds in FIG. 5).
Furthermore, evaluation of these compounds against several cancer cell lines revealed a marked and unexpected potent antiproliferative effect at extremely low concentrations.